Storage tanks for cryogenic fluids must be capable of maintaining the cryogenic fluid at extremely low temperatures (about -250.degree. F. to -150.degree. F.) and pressures (0.5 psig to 2 psig). Such tanks are used at liquefied natural gas storage sites and onboard ships. A pump to transfer the cryogenic fluid from the tank to another destination is typically capable of discharge pressures greater than 1000 psi.
If a large tank storing liquefied cryogenic fuel, such as liquefied natural gas, were to have a pipe line joined near the bottom of the tank, any pipe line failure could lead to the tank being emptied and the possibility that property in the vicinity could be damaged. To ensure that such a failure can not occur, discharge lines from cryogenic tanks exit the top of the tanks. With such an arrangement, it is necessary to place a high-pressure pump in the tank to pump liquid up the height of the tank through a discharge pipe.
To withstand high internal pressures, a discharge line must have thick walls. Further, to withstand the extreme cold temperatures of the cryogenic fluids the material used to make the pipe must be stainless steel or other cryogenic materials. Complicating the pump and discharge line arrangement in cryogenic storage tanks is that the pump requires electrical power and suitable electrical cables must be run into the tank and to the pump.
Further, a high-pressure pump must be disposed in the tank in such a manner that it can be removed for maintenance, repair, or replacement when necessary without draining the tank and taking the tank out of service, which can cost considerable time and resources.
One means for removing a pump from a cryogenic fluid storage tank includes a large diameter high-pressure pipe that acts both as a discharge line and a chamber for isolating the pump from the cryogenic tank contents during installation and removal. Failure to isolate the tank contents from an opening in the tank could result in the contents boiling off rapidly and escaping into the atmosphere. To isolate the contents, the high-pressure pipe includes at its lower end a foot valve that is normally closed but which can be opened simply by resting the pump on the foot valve. During installation, with the high-pressure pipe empty, the pump is lowered on a cable until it is near the bottom of the pipe. Before the valve is opened by setting the pump on the foot valve, a seal is placed near the top of the pipe so that the rush of cryogenic fluid into the pipe does not create a hazardous condition to the workers above. Once the seal is in place, the pump can be lowered to the bottom of the pipe and onto the valve to open the valve and permit the cryogenic fluid to flow into the pipe and be available to the pump.
To remove the pump, the high-pressure pipe is purged using inert gas at a pressure that is higher than that of the tank's contents. Once the high-pressure pipe is purged, the pump is raised by a cable to close the foot valve and seal the tank's contents from the high-pressure pipe.
Consequently, a high-pressure pipe for use in a cryogenic liquid storage tank must: extend from the top to the bottom of the tank; have a large enough internal diameter to accommodate the pump as it is installed and removed; have adequate strength to resist extremely high internal pressures; and be made of materials that can withstand the extreme cold of the cryogenic fluid. Such pipes are extremely expensive to build, maintain, and install.
A need exists for a method and apparatus for installing and removing a pump from a cryogenic liquid storage tank without the use of a large diameter, high-pressure discharge pipe.